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Flume investigation of the effects of sub-threshold rising flows on the entrainment of gravel beds

Piedra, Miguel M. (2010) Flume investigation of the effects of sub-threshold rising flows on the entrainment of gravel beds. PhD thesis, University of Glasgow.

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Abstract

Recent research on sediment transport in gravel bed rivers shows evidence of the influence of sub-threshold flow history on values of entrainment thresholds (Paphitis and Collins, 2005; Monteith and Pender, 2005; Haynes and Pender, 2007; Haynes and Ockelford, 2008). The research presented here analyses the effect of the characteristics of hydrograph rising limbs (flow magnitude and duration) on entrainment thresholds of gravel beds, with discharges ranging from 0.25-1.6 times the estimated bed threshold flow and durations from 0.5h to 6h. This analysis uses results from flume experimentation. Entrainment thresholds were determined by two well documented methods: a) particle movement counts (visual method, Yalin, 1977); and b) the reference transport method (RTM) (Parker et al., 1982a; Shvidchenko et al., 2001). Results obtained with near-uniform and uniform bed material sizes show a clear influence of flow magnitude and duration on entrainment thresholds, with bed resistance increasing up to c. 25% for longer durations of antecedent flows when using the visual method, similarly to Paphitis and Collins (2005). The results from the unimodal gravel bed suggest an intermediate duration of rising limb (c. 2h) producing the strongest bed, with more mobile beds resulting from both shorter and longer rising limbs. Total bedload transport rates reduce with increased bed resistance, this effect is also noted during the stability test phase. These results are used to develop a new simplified method for estimating critical bed shear stress using only total bedload data. The performance of a new formulation for bedload rates derived in this thesis is tested against a number of traditional bedload transport equations and appropriately discussed. In-depth analyses of bed surface and bedload size composition and surface grain structure show that bed surface undergoes little change of size composition, with a slightly proportionally larger reduction of fine content, suggesting penetration of fines below the surface. The analysis of coarse-grain bed surface structures, mobility and clustering, based on the size class containing D90 and using digital images taken under UV light, suggests that the surface distribution of coarse grains has a primary role on bed stability.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Research partly supported by Scottish Natural Heritage (SNH)
Keywords: entrainment thresholds, antecedent conditions, gravel bed rivers, sediment transport
Subjects: G Geography. Anthropology. Recreation > GB Physical geography
T Technology > TA Engineering (General). Civil engineering (General)
Colleges/Schools: College of Science and Engineering > School of Engineering > Infrastructure and Environment
Supervisor's Name: Haynes, Dr. Heather and Hoey, Prof. Trevor B.
Date of Award: 2010
Depositing User: Mr Miguel M Piedra
Unique ID: glathesis:2010-2134
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 12 Oct 2010
Last Modified: 10 Dec 2012 13:51
URI: http://theses.gla.ac.uk/id/eprint/2134

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